Many vegetable grain or seed products such as corn, sunflowers, and soybeans (generally referred to as oilseeds) have a substantial vegetable oil component. Often, this oil is extracted at some point while processing the raw products. The oil itself is a valuable commercial material used in foods, plastics, etc. The solids remaining after extracting the oil are also valuable and can be used for both human and animal foods, as well as for other purposes.
Early steps in processing change the form of the raw oilseed product to flakes or other types of particulate material. This particulate material is still permeated with most of the original natural oil. The oil is then extracted from this particulate material.
A number of different processes for removing or extracting the oil from this particulate material have been developed. The type of oil removal process of interest here is termed solvent extraction. After the grain has been converted to particles, the particles are immersed in a hydrocarbon fluid solvent such as hexane, heptane, isohexane, or any similar petroleum-based solvent that dissolves the oil.
Upon immersing the particles, the solvent forms a liquid solution with the oil in the particles. The oil-solvent solution is then removed from the particles in some manner, usually by simple gravity draining. In gravity draining, a screen supports the particulate material and allows the oil-solvent solution to drain through the screen to a catch basin. The solvent and oil are then separated with a conventional process. Usually, the solvent recovered during this separation step can be used again in the extraction process.
After the oil-solvent solution has drained from the particulate material, there is usually still a significant amount of solvent and a smaller amount of oil permeating the particulate material. Where the particulate material will be used as human food or animal feed, it is important for a number of reasons to remove nearly all of the solvent from the particulate material. First, the solvent may be toxic, so removing the solvent from the particulate material prevents harm to whomever or whatever might consume the end product of the process. Secondly, whether the solvent is toxic or not, it may be an air pollutant so it's important to prevent as much of the solvent as possible from reaching the atmosphere. Third, the solvent is valuable. Extracting it from the particulate material allows its reuse in the oil extraction process.
U.S. Pat. No. 5,630,911 (Kratochwill) discloses apparatus and process for removing a substantial amount of the remaining solvent following gravity draining or other type of oil-solvent removal. The Kratochwill apparatus uses, within an enclosed vessel or volume, a number of inclined conveyors that carry the particulate material over heating plates. The particulate material permeated by the solvent still present is heated to vaporize the solvent. This solvent vapor can then be removed from the enclosed space. Some oil remains in the particulate material, but it forms a small percentage of the total mass. Kratochwill is incorporated by reference into this application.
One feature of the Kratochwill apparatus is that the process occurs at a temperature high enough to reduce the protein dispersability index (PDI) of particulate material having high protein content. A high PDI is preferred for some processed oilseed materials; for these materials, lower process temperature is an advantage.